This invention relates to improvements in coating porous webs such as webs of paper and the like and more particularly relates to a method and apparatus for doublewet coating a web of paper wherein a blade coater may be used to smooth a first relatively high viscosity coating layer to which is directly applied while such layer is wet, and at the same station, a relatively low viscosity coating layer which may then be smoothed by an air knife.

The so'called air-brush or air-knife method of coating is disclosed, for example, in United States Patent No. 1,980,923 to Label and in improved detail in United States Patent No. 2,139,628 to Terry. It is known that such an air-knife coating apparatus which normally comprises means such as a roller or backup roll for supporting one side of the web and means to apply a substantially nondivergent jet of air emitted from a straight slit orifice of a nozzle to the coating on the web tends to leave a uniformly thick layer of coating on the web. In practice, the jet of air issues from the nozzle with substantially uniform velocity throughout its entire extent and impinges upon the wet coating at an angle and in a direction opposite to the direction of web travel, thereby cutting away the excess coating and leaving the desired quantity of coating on the web in a layer of uniform thickness. This characteristic has been used to produce semi-coated grades of paper in accordance with the teachings of, for example, US. Patent No. 2,325,798 to L. W'. Porter. As indicated in Porter, the type of fluid coating composition suitable for use therein and in the instant invention is one in which a solid component is suspended in a liquid carrier or vehicle. The instant invention will be described in connetcion with coating a paper body-stock with a mineral-coating composition that is a fluid composition comprising a pigment component (e.g., clay, calcium carbonate, ochre, aluminum powder, satin white, titanium dioxide and the like) and an adhesive component (cg, starch, casein, vegetable protein, or equivalent) suspended in an aqueous vehicle.

The finished quality of any coated paper prepared with an air-knife coater, however, depends to a considerable extent on the quality of the paper sheet or web. In particular, the air jet will remove coating from the cavities and pores of the base sheet surface with the result that such defects are also found in the surface of the coated sheet. The very characteristic which gives the air jet technique one of its advantages leads also to this disadvantage. Inasmuch as the air jet produces, as noted above, a uniformly thick layer of coating, it follows that any irregularities in the base sheet or web will also be reproduced in the surface of the coated paper where a single coating layer is smoothed by air jet techniques. On the other hand, multiple layer coating techniques requiring a drying step between the application of successive layers in order to achieve a smooth surfaced paper unduly increase the cost of the final product.

Another problem of particular significance in the coating art resides in the fact that many coating compositions that may be easily applied to a paper web develop weaknesses or imperfections therein upon drying such that these coatings tend to fail in their adhesion to the web and separate therefrom. This has been found to be par- 3,22,53h Patented Aug. 24, 1965 ticularly true in the case of relatively low viscosity (i.e., low solids content) coatings.

It is, therefore, an important object of the instant invention to provide an improved method and apparatus for coating paper which overcomes the above noted difliculties in the art.

It is a further object of the instant invention to provide a method and apparatus for coating paper by which two successive layers of coating may be applied at a single coating station without an intermediate drying step.

It is a further object of this invention to provide a method and apparatus of coating paper wherein an airlmife may be used to remove the excess of the final layer of coating without affecting the earlier layer or layers and without reproducing web irregularities in the final surface.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawings attached hereto and made a part hereof.

On the drawings:

FIGURE 1 is a diagrammatic illustration of a first embodiment of apparatus suitable for carrying out the method of the present invention;

FIGURE 2 is a diagrammatic illustration of a second embodiment of apparatus suitable for carrying out the method of the present invention;

FIGURE 3 is an essentially diagrammatic fragmentary elevational view showing the paper web of the invention at various stages of the coating process; and

FIGURE 4 is still another diagrammatic illustration of .a third embodiment of apparatus suitable for carrying out the method of the present invention.

As shown on the drawings:

Although it i not desired to limit the invention to any particular theory, reference is made initially to the diagrammatic showing of FIGURE 3 to explain what are believed to be the theoretical considerations involved in distinguishing the instant coating method and paper coated thereby from the prior art. In FIGURE 3, on the left hand side the reference numeral 19 indicates generally the paper web to be coated at an initial stage in the coating process and the reference numeral 10a on the right hand side of FIGURE 3 indicates the same paper web in a subsequent coating step. The initial coating indicated generally by the reference numeral 11 is here represented by a plurality of small circles representing diagrammatically the solid particles that are suspended in the aqueous solution of adhesive such as starch, casein, and the like. The initial coating 11 may be applied to the surface of the paper 10 by means of a blade coater shown fragmentarily at 12, which generally serves to fill the valleys or surface irregularities indicated diagrammatically at 13 and 1 5 in the Web 10 with the coating 11 and present a relatively smooth surface 15 for the outer surface of the coating .11. The blade 12 may be the functioning part of a typical trailing blade coater, which is capable of applying coatings throughout a substantial range of viscosities, including the rather high viscosity coatings. Even with relatively viscous coating compositions 11, however, it has been found that the trailing blade 12 or other mechanical smoothing means may result in the formation of a relatively smooth surface 15, while still substantially eliminating the effect of the rather great surface irregularities 13, 14 in the web 10. This i the way in which the instant invention is carried out, using a coating composition 11 of relatively high viscosity to form the initial layer on the Web 16 with a comparatively smooth surface 15, although not necessarily a surface having the characteristics desired for top quality publication grade papers or the like.

if, in contrast, a coating composition 11 of relatively low viscosity is applied to the initial uncoated web 10, in accordance with the practice of the prior art, it has been found that when the coating 11 ultimately dries, there is a tendency for the coating to fail along a plane indicated generally at 16 that is very closely adjacent to the approximate plane of the interface (indicated generally at 17) between the web and the coating "11.

rather substantial migration of the aqueous phase of the coating composition 11. In order words, it is believed that as soon as the coating 11 is applied to the web 10 capillary action (or related phenomenaltends. to draw :a substantial portion of the aqueous system or vehicle carrying the solid particles in the coating ll'into the body of the Web 10 in an area designated in dot-ted *lines generally at 18. The aqueous system drawn into layer 11, but the water thus migrating back to be evaporated by suitable drying means does not carry back into the coating layer any appreciable portion of the very substantial quantity of adhesive which it initially conveyed into the web area 18. This it is believed results in a shortage-of adhesive in a relatively localized 'area,'-i.e., along the plane 16 in the coating 11, o that the coating tends to fail.

It has been found 'tha the phenomenon of the weakness of the dried coating layer along the plane 16 is reduced, if either the viscosity or th solids content of This failure is now believed to be caused by a It is believed that during the ultimate drying of the coating is increased. The viscosity may be increased merely by increasing the solids content or it may be increased by the incorporation in the aqueous system of materials which themselves have a relatively higher viscosity or whichform an aqueous system of higher viscosity when dissolved in the aqueous phase of the coating. It has been found that the phenomenon of the tendency toward weakness at the plane 16 in the dried coating isreduced if the initial layer of coating is high viscosity coating and a subsequent layer of lower viscosity coating is applied over the first layer while it is wet.

If it is attempted to apply relatively substantial quantities of the high viscosity coating to the sheet, the blade '12 must be applied to the sheet with such low forces that the control of .the coating process is difiicult. In addition scratches and other defects which are commonly experienced during coating with smoothing or wiping devices mar the surface of the coated paper. The application of only sufficient coating in the first layer to provide the smoothing or leveling function minimizes the depth or size of these'defects and permits substantial coverage of such defects by the application :of a second layer of lower viscosity cola-ting. Thus, one is able to apply the desired larger total quantity of coating to the sheet without the disadvantages which are present in attempting to apply the same amount of coating either completely as high viscosity coating with a blade coating device or completely with an air-knife coating apparatus and low viscosity coating.

As previously indicated, in accordance with the present invention, a double-wet coating process is used to avoid the problems of the prior art noted above. In accordance with this process, a first layer of a fluid or liquid coating composition of relatively high viscosity is applied to the traveling paper. web (using, for example, a publication grade web) to be coated. The fluid coating of this first layer, (indicated after it is smoothed at 11a in FIGURE 3) may conveniently comprise a liquid in which is suspended about 55 to solids by weight. Such a percentage of coating color produces a coating which preferably weighs between 3 to 8 pounds per ream, i.e., 3 to 8 pounds of coating per 3300 square feet of paper web. In actual practice, the weight of the coating may range from a minimum of about 3 pounds per ream to a practical maximum of about 10 pounds per ream, above which the difliculties hereinbefore referred to are encountered. This first layer of coating 11a may be applied conveniently and smoothed with a conventional blade coater (e.g. a trailing blade coater) positioned in contact with the side of the traveling web to be coated, while the other side is supported firmly, for example, by a backup roll or similar supporting means.

Although it is conventional in the paper making art to correlate solids content and viscosity and evenexpress the viscosity in terms of the solids content, it is possible to introduce some variation in the viscosity of a coating composition of given solids content by the introduction of materials into the aqueous system which will alter the viscosity. This may be done by conventional thickening agents, or it may even be done by variations in the actual quantity and/or physical properties of the adhesives that are dissolved or dispersed in the aqueous system. In the practice of the instant invention it has been found that the initial coating composition 11 that is applied by a blade coater 12 should have a minimum viscosity of at least about 4000 centipoises (as measured with a Brookfield viscometer at a rotational speed of 6 r.p.m., which is the basis for all of the viscosity figures'hereinafter described). Coatings of this viscosity or more will be found to be sutficiently viscous to avoid being influenced by the air-knife jet. The upper limit .of'the viscosities which may be used in the initial coating composition 11 is a practical limit and, if desired, coatings of up to 40,000 centipoises viscosity may be used in the initial blade coating step.

A specific example of the coating composition 11 used in the practice of the instant invention has the following formulation:

parts of clay 16 parts of soya protein 2.4 parts of 28% ammonium hydroxide 1 part of calcium stearate 1 84 parts of water a avoiding the mottled appearance which can result because of the application of non-uniform quantities of coating over a sheet surface to obtain a relatively smooth exposed coated surface. In the practice of the invention, such a coating couldbe used as the initial coating 11 on a kraft paper to obtain a smooth surface 15 of uniform color (which, in turn, could then be coated by a coating of any particular color desired without the formation of a motside), which may be conveniently smoothed by the airknife so as to obtain a very high quality smooth surface 1%. The use of a higher solids coating in the first layer 11a followed by a lower solids coating 19 in the second layer enables one to enjoy the advantages of the air-knife coating process without suffering the poorer bonding of coating to fiber that results with low solids coatings. It is possible, therefore, to reduce the amount of adhesive in the top coating layer without suffering reduction in the bonding strength. This has been found to be particularly advantageous in the production of coated papers of the invention which have improved receptivity to printing inks.

The coating composition 19 has a relatively low vis cosity and, therefore, relatively low solids content, such that if it were applied in place of coating 11 as the initial layer the migration of adhesive into the web would result in a plane of weakness in the dried coating film for the reasons heretofore described. In contrast, however, when this low viscosity coating 19 is applied to the relatively smooth surface 15 of the still wet higher viscosity coating Ila, the coating layer lilo functions as a barrier to resist the rapid migration of the aqueous component from the composition I9 and into the web 1051. Instead, it is believed that the initial highly viscous coating 11 yields a relatively smaller amount of its aqueous component to the web ill-lilo which perhaps may be indicated by penetration only to the depth of the heavy-line 18a. In turn, the comparatively substantial aque us component of the lower viscosity coating composition l? is not exposed directly to the capil ary or other adsorption action of the web because the initial coating layer Ila is interposed therebetwecn. The attraction for water of the fibers of the web 1% is believed to be substantially greater than the ability of the particles in the coating layers Illa or to retain water, but the coating layers 11a and 19 contain particles having an approximately equal attraction for water, so that the aqueous component of the layer 19 may be partially adsorbed or retained by the viscous layer llrz (if the water content thereof has been appreciably depleted by the web llda), but that is approximately the extent to which any migration of the queens component of the composition 19 takes place. Also, this rather limited type of water migration does not create any weakness in the coating layer 19a closely adjacent the interface at the smooth layer 15.

The second layer 19a of coating is applied to the coated sheet or web without any intermediate drying of the layer 11a and preferably at the same coating station at which the first layer 11a was applied. This second coating is preferably of about to by Weight in solid con tent and preferably has a viscosity which is relatively low by comparison to the viscosity of the first layer 11a so that the second coating layer 19a may be readily handled in an air-knife coater.

A specific example of a second coating composition used in the practice of the instant invention has the following formulation:

90 parts of clay 10 parts of titanium dioxide 4 parts of casein 12 parts of latex adhesive (Dow Latex 512L-solids basis) 06 part ammonium hydroxide (28%) 2 parts ammonium stearate 156 parts water The above coating formulation has a solids content of about 43% and a viscosity of approximately 550 centipoises. The viscosity used for the second coating 19 in the practice of the instant invention is limited practically to the viscosity of coatings which can be readily handled by the air-knife 2t). Such viscosities are within the range of about 100 to about 1500 centipoises.

In the use of the specific devices shown in FIGURES l, 2 and 4 herein, it will be understood that the initial coating has a specific formulation for the coating 11 hereinbefore described and the subsequent coating layer has the specific formulation for the coating 1) just described. In addition, the initial coating Ill is applied in a Weight of approximately 5 pounds per ream, and the subsequent coating layer 1% is applied in a coating weight of approximately '7 pounds per ream. In the practice of the invention, the second coating layer 19a may be applied in coating weights ranging from 3 to 14 pounds per ream.

In the practice of the instant invention the air-knife 26 of the type referred to in the previously mentioned prior art patents is positioned to remove the excess of coating material 19 so as to obtain the desired smooth second layer 3%. The coating process is itself a doublewet coating process, and it will be appreciated that the air-knife causes no practical amount of drying itself in carrying out its function. The air-knife 2t) merely re moves the excess over and beyond that desired. In addition, the air-knife 2th has a negligible effect upon the higher solid content of the first layer Ila. The first layer Illa is applied and smoothed so as to obtain a relatively smooth surface 15. The coating composition 19 is applied by means which will be described in detail hereinafter in such a manner so as not to effect substantial intermixing between the layers llla and 19a, and the air jet 20 smooths the layer Ha as to obtain an extremely high degree of smoothness, again without substantiall' intermixing the layers flu and I911. Since the air jet 20 acts primarily only On the second lower viscosity layer 19 which is spread over the already smoothed surface 15 of the first higher viscosity layer Illa, the leveling obtained in applying the first coating layer Ila permits the preparation of a much more leveled and highly smoothed surface 21 on the layer 19a after the air-knife step. As indicated, both coating layers may be applied at the same coating station, and even against a web traveling over the same backup roll. In the present process, the air jet 20 does not remove the coating material from cavities or pores of the web base sheet surface as in the Case of the prior art because such irregularities in the web have been filled by a relatively high viscosity first coating layer 11a which has been smoothed to provide a smooth surface 15 on which the relatively low viscosity second coating layer 1941 may be applied in uniform thickness by the air-knife directly, even while the first coating layer 11a is still wet.

Several possible arrangements of apparatus suitable for carrying out the present invention may be used.

Referring to FIGURE 1, a web 3a} which is to be coated i trained around a reversing roll 31 and a supporting roll or backup roll 32.. The first coating layer 11 may be applied by a duct or blade coater 33, the blade of which is positioned closely adiacent the surface of the backup roll 32 which firmly supports the back or uncoated side of the web The coater 33 may be of the type disclosed in Trist US. Patent No. 2,368,176.

It should be appreciated that the trailing blade coater 33 is of the type wherein a stationary element having a straight and. sharp trailing edge 33a acts in a wiping manner on the surface of the web 10. Coaters of this type have the capability of developing a maximum smoothing of the coating on the sheet. The blade coater of Trist is an excellent example of this type of device.

The coating material may be supplied to the trailing blade coater 33 in any convenient or conventional man ner such as from a trough at the bottom of which the blade 33a is mounted. The web 30 may next be trained over a coating roller or applicator roll 34 which is mounted to dip in a trough 35 containing the fluid coating suspension 19 to be applied as the second Coating layer 19a. It will be noted that the same side of the web 36 on which the first coating layer Illa was applied by the blade 33:: is contacted by the surface of the roller 34 to apply the second layer 19 thereto. The coating or applicator roller 34 is a conventional type of coating device and may be of the type disclosed in US. Patent No. 2,508,287, in greater detail. it will be noted that the applicator roll 34 is mounted closely adjacent to the trailing blade coater V side of the web 30 in a position to be acted upon by the.

air jet from the air-knife arrangement 37. The air-knife 37 (as well as the air-knife previously shown diagrammatically) may be of the conventional type hereinbefore discussed and shown in detail in the prior art and will act to remove the excess of the second coating composition 19 so as to obtain the desired smooth coating layer 19a.

Although the arrangement shown in FIGURE 1 is such that the second layer of coating 19 is applied to the web after it leaves the first supporting roll or backup roll 32, it will be seen from FIGURE 2 that both coatings may be applied and smoothed while the web is supported by a single roll.

Referring now to FIGURE 2, it will be noted that parts corresponding to the parts shown in FIGURE '1 are indicated by the same reference numeral in the 40 series. It will be noted in FIGURE 2 that the web 4% is trained over a reversing roll 41 and then between the trailing blade coater 43 and the supporting or backup roll .42, comparable to the arrangement shown in FIGURE 1. The applicator roll 44 which applies the second relatively low viscosity coating layer 19 is positioned in FIGURE 2 immediately downstream from the trailing blade coater 43 and forms a nip with the backup roll 12. The appli cator roll 44 is positioned in close running relationship to the backup roll 42. It may be spaced from the backup roll beyond the thickness of the web or it may be run with a very low nip loading. This is to avoid compacting or compressing of the web 40 at the second coating position to the extent that the first coating layer is disturbed. The roller coater 44 mounted submerged in the coating bath in the trough 45 thus applies the coating composition 19 as the second layer to the web 40 passing through the nip. Operation of the roll coater 44 without conventional roller coater nip loads is necessary to avoid substantially intermixing the layers 11a and 19. Operation of the roller coater in this manner results in the application of relatively large quantities of coating to the sheet beyond such quantities as would be applied by roll coaters used to produce coated papers of commerce. The details of the mounting of the roller coater 44 are, however, brought out in previously mentioned US. Patent No. 2,508,287.

It will, of course, be understood that all of the rolls herein described may be driven by any conventional means known to the art at suitable speeds and that the pressure between the specific rolls 42 and 44 may be controlled in any conventional manner known to the art.

Next, an lair-knife 47 is positioned in close running relation to the backup roll 42 but again spaced slightly therefrom beyond the thickness of the web and it is also.

positioned closely adjacent to the applicator roll 44. The air-knife 47 applies a jet of air to the web 40 after it leaves the nip between the rolls 42 and 44 and before it is separated from the surface of the roll 42. Again, the air jet from the knife 47 functions to remove excess quantities of the second coating layer 19 applied to the web by the applicator roll 44. If desired, the trough 45 may be ex tended under the point of contact of the air jet with the web to receive such excess coating material.

Referring now to FIGURE 4, it will be seen that a somewhat different arrangement is shown, In FIGURE 4 the reference numerals designating parts corresponding to parts already described have the same number in the 50 series. a For example, the continuously advancing web is designated by the reference numeral and it passes between a conventional backup roller 52 and a trailing 8 blade coater 53 wheie'at the initial coating layer 1111 is applied and smoothed. Immediately adjacent to this ini tial coating device there is positioned an applicator roller 54 submerged in the coating bath in a trough $5 so that it may apply the second coating 19 to the surface of the still wet initial coating layer 11a. The coated web Sil then passes over a second backup roller 56. Again, an air-knife 57 is mounted in close proximity to the applicator roller 54 and in close running relation to the backup roller 56 so as to remove excess of coating to obtain the ultimate desired thickness for the second coating layer In FIGURE 4, however, there is added an additional smoothing and spreading element in the form of a doctor blade 53 that is positioned immediately ahead of the airknife 57 and downstream from the applicator roll 54. The blade 5-8 serves as a mechanical smoothing and spreading device for the second coating layer 19 so as to remove a portion of the excess of this coating composition (such portion being designated at 1%). The doctor 53- and the coating bath trough 55 are so mounted that this portion of the excess 1% may flow back into the coating bath in the trough 55. The remainder of the eXCess is, however, removed by. the air-knife 57. This results in the extremely high quality smooth surface hereinbefore referred to in connection withFiGURE 3 as 21. This remainder of the excess is indicated diagrammatically at 19c in FIGURE 4 and itis shown being collected in a secondary trough 55a and returned through a suitable conduit'59 to the main trough 55.

7 It will be appreciated that the very light touch resulting in the high degree of smoothness that is obtained using the air jet cannot ordinarily be duplicated by the use of a blade, such as the blade 58. The operation of the blade is not ordinarily that delicate. On the other hand, the blade 58 may be used to remove a substantial portion of the excess 1% and thus simplify the final smoothing operation of the air-knife 57. The arrangement in FIGURE 4 permits the application of the maximum amount of coating in the second layer 19 by the applicator roll 54. Initial smoothing of this maximum amount of coating may then be carried out by the blade 58 and final delicate smoothing may be carried out by the air-knife 57. It will be appreciated, however, that the blade 58 is operated with a delicate wiping touch so as to avoid substantial intermixing of the layers 11a and 19 at this position.

Nothwithstanding the misconceptions of the prior art, the instant invention is predicated upon the discovery that the usual intermediate drying step in plural coating operations is not required; Moreover, intermixing of the coating layers 11a and 19 which might have been expected on the basis of prior art knowledge does not occur to any significant degree in the practice of the instant invention. This is'primarily becausethe air-knife operation is controlled by virtue of the use of an initial highly viscous layer 11a that is not appreciably effected by the air-knife and the use of a second low viscosity layer 19 which is readily effected by the air-knife By' simple adjustment of the air-knife, a very smooth level second coating 19a is achieved, which is superior to that obtained by known single coating methods or plural coating methods involving intermediate drying steps.

In addition, the instant double-wet coating process using the final air-knife coating step results in an overall lower drying load. As indicated previously, the drying effect of the air-knife per se is negligible; and drying is carried out subsequently to the air-knife step. As an example of the advantagein this drying process, however, if 6 pounds per ream of dry coating were applied to a sheet in the form of a 60% solids composition and -an additional equal amount of coating were added as a second layer in the form of a-40% solids coating composition, the total water applied to the web would be only about 72% of that added, if a similar total amount of dry coating were added entirely in the form of a 40% solids content composition.

As previously indicated herein, the applicator rollers 34, 44, 54 are rotatable. They may be rotated in either direction, but preferably they are rotated in the direction of travel by any siutable drive means such as the drive motor M indicated diagrammatically in FIGURE 4. Also, it has been found preferable to drive the applicator rolls 34, 4-4, 54 (and particularly the nip-defining roll 44) at a speed sufiicient to load the oncoming side of the coating roll contact with the web, i.e., to load the nip with the roll 44 so that a level of coating bath is maintained at least slightly above the nip level at the oncoming side of the nip. This is not the normal bath level (unless a submerged nip is used) but it is a level (at L) in the immediate vicinity of the oncoming side of the nip that is maintained by driving the roll 44 at the necessary speed.

It should also be noted, although the instant invention is directed primarily to the double-wet coating process, a single layer coater may be used in the form of the embodiment of FIGURE 4 comprising only the applicator roll 54 (which may be in nip-defining relationship with the backup roll 56 in the manner of the rolls 44, 42 of FIG- URE 2, or which may be as shown in FIGURE 4), the blade 58, and air-knife or doctor 57 all operating in conjunction with the backup roll 56. This single layer coater system could be used in the absence of the earlier trailing blade coater 53 to apply only one coating to the web.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

1 claim as my invention:

1. A method of double-wet coating one side of a flexible continuously advancing web while firmly supporting the other side at a single coating station comprising, mechanically spreading on said one side of said web a first layer of a relatively high viscosity fluid coating comprising about 55% to 65% by weight of solids suspended in a liquid to provide a smooth surface on said one side of said web, applying to without substantially intermixing with said smoothly spread first layer while it is wet a second layer of a relatively low viscosity fluid coating comprising about 30% to 45% by weight of solids suspended in a liquid, and applying a jet of air to said coated web to remove only the excess of said second coating without substantially intermixing said coatings and leave a residue of said second coating which is of uniform thickness above said first smooth coating and which consequently affords a smooth outer surface on said web irrespective of irregularities in the original surface of said web.

2. A method of double-wet coating one side of a web while firmly supporting the other side comprising, mechanically spreading on said one side of said web a first layer of a relatively high viscosity fiuid coating composition to provide a smooth surface on said one side, applying to without substantially intermixing with said smoothly spread first layer while it is wet a second layer of a relatively low viscosity fluid coating composition, and applying a jet of air to said coated web to remove only the excess of said second coating without substantially intermixing said coatings and leave a residue of said second coating which is of uniform thickness above said first smooth coating and which consequently affords a smooth outer surface irrespective of irregularities in the surface of said Web.

3. A method of double-wet coating one side of a flexible web while the web is being firmly supported at and continuously advanced through a single coating station comprising, smoothly spreading on said one side of said web a first layer of a relatively high viscosity fluid coating composition comprising a liquid and a suspended solid, thereafter applying to without substantially intermixing with said smoothly spread first layer a second layer of a relatively low viscosity fluid coating composition comprising a liquid and a suspended solid, and applying a jet of air to said coated web to remove only the excess of said second coating without substantially intermixing said coatings and leave a residue of said second coating which is of uniform thickness above said coating and Which consequently alfords a smooth outer surface on said web irrespective of irregularities in the original surface of said web.

4. A method of producing from a web having an irregular surface a coated paper having on at least one side a smooth surface coating layer of uniform composition and of uniform depth comprising, spreading on said one side of said web a first layer of a relatively high viscosity fluid coating composition to provide a smooth surface on one side, applying to without substantially intennixing with said smoothly spread first layer While it is wet a second layer of a relatively low viscosity fiuid coating composition, and then applying an air jet to the second layer for removing only the excess of said second coating layer Without disturbing said first coating layer to leave a residue of said second coating which is of substantially uniform thickness above said first smooth coating and which consequently affords a smooth outer surface irrespective of irregularities in the surface of said Web.

5. A method of double-wet coating one side of a continuously advancing paper Web, which comprises the steps of mechanically spreading on one side of said web a first layer of a liquid coating having a minimum viscosity of 4000 centipoises and having a solids content of 55% to 65% to fill in surface irregularities on said Web and present a smooth outer surface for said first layer, next applying to without substantially intermixing with said smoothly spread first layer while it is wet a second layer of liquid coating having a viscosity of to 1500 centipoises and having a solids content of 30% to 45%, and apply a jet of air to said coated web to remove only excess of said second layer while it is wet and without substantially intermixing said layers and to leave a residue of said second layer which is of substantially uniform thickness and which presents a smooth outer surface irrespective of irregularities on the original web surface.

6. A method of double-wet coating one side of a continuously advancing paper web, which comprises the steps of mechanically spreading on one side of said Web a first layer of a liquid coating having a minimum viscosity of 4000 centipoises to fill in surface irregularities on said web and present a smooth outer surface for said first layer, next applying to without substantially intermixing with said smoothly spread first layer while it is wet a second layer of liquid coating having a viscosity of 100 to 1500 centipoises, and apply a jet of air to said coated web to remove only excess of said second layer while it is wet and without substantially intermixing said layers and to leave a residue of said second layer which is of substantially uniform thickness and which presents a smooth outer surface irrespective of irregularities on the original web surface.

7. A method of double-wet coating one side of a continuously advancing paper web, which comprises the steps of mechanically spreading on one side of said web a first layer of a liquid coating having a minimum viscosity of 4000 centipoises to fill in surface irregularities on said web and present a smooth outer surface for said first layer, next applying to without substantially intermixing with said smoothly spread first layer while it is wet a second layer of liquid coating having a viscosity of 100 to 1500 centipoises, and apply a jet of air to said coated Web to remove only excess of said second layer while it is wet and without substantially intermixing said layers and to leave a residue of said second layer which is of substantially uniform thickness and which presents a smooth outer surface irrespective of irregularities on the original web surface, said steps being carried out while the other side of said web is firmly supported by single rigid rotary surface.

8. A method of double-wet coating one side of a continuously advancing paper web, which comprises the steps of mechanically spreading on one side of said web a first layer of a liquid coating having a minimum viscosity of 4000 cenitpoises and having a solids content of 55% to 65% to fill in surface irregularities on said web and present a smooth outer surface for said first layer, next applying to without substantially intermixing with said smoothly spread first layer while it is Wet a second layer of liquid coating having a viscosity of 100 to 1500 centipoises and having a solids content of 30% to 45%, then mechanically removing a portion of excess from said second layer without substantially intermixing said layers, and apply a jet of air to said coated Web to remove the remainder of excess of said second layer while it is wet and without substantially intermixing said layers and to leave a residue of said second layer which is of substantially uniform thickness and which presents a smooth outer surface irrespective of irregularities on the original web surface.

9. Apparatus for double-wet coating one side of a Web, comprising a backup roll engaging the other side of and firmly supporting a continuously advancing web, mechanical spreading means in close running relation to said backuproll for applying a first layer. of relatively high viscosity coating to the web to provide a smooth coated surface on one side of the web, second means positioned closely adjacent said spreading means and in close running relation to said backup roll but spaced slightly therefrom beyond the thickness of the web for applying a second layer. of relatively low viscosity coating to said first layer while it is wet without substantially intermixing the layers, and air-knife means in close running relation to said backup roll applying a jet of air to said second layer to remove only excess of said second coating with: out substantially intermixing said layers and to leave aresidue of said second layer which is of substantially uniform' thickness'and which presents a smooth outer surface irrespective of irregularities on the original web surface. i V V i 10. Apparatus for double-wet coating one side of a web, comprising a backup roll engaging the other side of and firmly supporting a continuously advancing web, mechanical spreading means in close running relation to said backup roll for applying a first layer of relatively high viscosity coating to the web to provide a smooth coated surface on one side of the web, an applicator roll positioned closely adjacent said spreading means and in close running relation to said backup roll but spaced slightly therefrom beyond the thickness of the web for applying a second layer of relatively low viscosity coating to said first layer while it is Wet Without substantiallyintermixing the layers, and air-knife means in close running relation to said backup roll applying a jet of air to said second layer to remove only excess of said second coating without substantially intermixing said layers and to leave a residue of said second layer which is of substantially uniform thickness and which presents a smooth outer surface irrespective of irregularities on the original web surface.